The present invention relates to the art of press-fitting two members with each other and particularly to the art of applying a kinetic energy to at least one of two members in a direction which causes the two members to approach each other, and thereby press-fitting the two members with each other.
The above-indicated press-fit art is the art of utilizing inertia of at least one of two members and press-fitting the two members with each other by impact thereof, and is disclosed in Japanese Patent Application laid open under Publication No. 9-66421.
Generally, the inertia-press-fit art requires the art of stabilizing a relative position of two members at a time when the press-fitting of the two members ends (hereinafter, referred to simply as the xe2x80x9cpress-fit-end positionxe2x80x9d). To meet this requirement, the present inventors had proposed, in the inertia-press-fit art, the art of defining the press-fit-end position of two members by causing the two members to butt each other.
However, if, in the above-indicated art, at least one of the two members has a remaining kinetic energy immediately after the butting of the two members, the two members are rebounded from each other, so that an actual press-fit-end position of the two members is deviated from a normal position.
The present invention has been developed in the above-indicated background, and an object thereof is to provide an inertia-press-fit method which can sufficiently stabilize the press-fit-end position of two members.
The above-indicated object is achieved according to each of the following features of the present invention. Those features are given respective sequential numbers and are described, as needed, in the same manner as that in which claims are described, i.e., any feature that includes another feature shall do so by referring to the number given to the latter feature. This is for helping more easily understand that two or more of the following features may be combined. It is to be understood that the technical features, and the combinations thereof, described in the present specification are not limited to the following features and the combinations thereof.
(1) An inertia-press-fit method of applying a kinetic energy to at least one of two members in a direction which causes the two members to approach each other, and press-fitting the two members with each other, characterized by
causing the two members to butt each other to define a relative position of the two members at a time when the press-fitting ends and, immediately after the butting, causing at least one of the two members to plastically deform and thereby absorb a remaining kinetic energy of the one member.
In the present method, the remaining kinetic energy of the one member (to which the kinetic energy had been applied), at a time immediate after the butting of the two members, is absorbed into plastic-deformation energy by at least one of the two members. Therefore, in the present method, even if at least one of the two members may have a remaining kinetic energy immediately after the butting of the two members, an actual press-fit-end position of the two members is not largely deviated from a normal position.
Each of the two members may be provided by a single part or by an assembly consisting of a plurality of parts. In the latter case, the plurality of parts may include such a part which is used exclusively when the press-fitting is performed. The latter part may be a buffer member which, immediately after one of the two members to which the buffer member belongs butts the other member, plastically deforms and thereby absorbs the kinetic energy (i.e., excessive kinetic energy) which remains immediately after the butting.
(2) An inertia-press-fit method according to the feature (1), wherein at least one of the two members includes a butting portion which butts the other member, and an interference-fitting portion which interference-fits with the other member, the butting portion being more plastically deformable than the interference-fitting portion.
In this method, the butting portion contributes to absorbing the kinetic energy which remains immediately after the butting, thereby stabilizing the press-fit-end position of the two members. In addition, since the interference-fitting portion provides an anchoring effect by mechanically engaging the other member, the two members are bonded with each other with an increased strength.
(3) An inertia-press-fit method according to the feature (2), wherein the butting portion of the at least one of the two members has a Vickers hardness Hv smaller than that of the interference-fitting portion thereof.
Generally, hardness of a material has a positive correlation with yield stress. More specifically described, as hardness decreases, yield stress also decreases. As yield stress of a material decreases, the material is more likely to plastically deform and thereby absorb an external energy into plastic-deformation energy. Therefore, a material with a lower hardness is more likely to absorb an external energy. In addition, hardness of a material has a positive correlation with coefficient of rebound. More specifically described, as hardness decreases, coefficient of rebound decreases. As coefficient of rebound of a material decreases, amount of rebound of the material upon butting on another member decreases. Meanwhile, there are various hardness measuring methods one of which is known as Vickers harness. Based on those knowledges, the method according to the present feature (3) has been developed as one embodiment of the method according to the feature (2).
(4) An inertia-press-fit method according to any one of the features (1) to (3), wherein the two members include respective butting portions which butt each other, at least one of the butting portions having a shape which allows a material thereof to flow and thus plastically deform.
In the present method, the shape factor of at least one of the two butting portions assures the plastic deformation, which leads to absorbing efficiently the excessive kinetic energy immediate after the butting.
(5) An inertia-press-fit method according to any one of the features (1) to (4), wherein the two members are caused to butt each other at a stopper surface inclined relative to a plane perpendicular to the direction, to define the relative position of the two members at the time when the press-fitting ends.
In the case where the press-fit-end position of the two members are defined by causing the two members to butt each other at a stopper surface perpendicular to the direction in which the two members approach each other, a rebounding force whose magnitude is equal to that of the excessive force acting from one member to the other member immediately after the butting, acts from the other member to the one member in a direction opposite to the direction in which the excessive force acts. In contrast, in the case where the press-fit-end position of the two members are defined by causing the two members to butt each other at a stopper surface inclined relative to a plane perpendicular to the direction in which the two members approach each other, a rebounding force whose magnitude is smaller than that of the excessive force acting from one member to the other member immediately after the butting, acts from the other member to the one member in the direction opposite to the direction in which the excessive force acts. Thus, under the condition that the excessive force is same in the two cases, the rebounding force received, in the latter case, by the one member from the other member immediately after the butting is smaller than that received in the former case by the one member, so that the distance by which the one member is rebounded, in the latter case, from the other member with the excessive force immediate after the butting is smaller than that in the former case.
Based on the above-indicated knowledges, in the method according to the present feature, the press-fit-end position of the two members are defined by causing the two members to butt each other at the stopper surface inclined relative to the plane perpendicular to the direction in which the two members approach each other. Therefore, the present method assures that, if the kinetic energy remains immediately after the butting, the press-fit-end position of the two members is also stabilized by the mechanism that the rebounding force is decreased by the shape factor of at least one of the two members.
(6) An inertia-press-fit method according to any one of the features (1) to (5), wherein one of the two members is a torsion bar which is used in a power steering device of an automotive vehicle and the other member is a shaft which is used in the power steering device and which is press-fitted in the torsion bar, and wherein the torsion bar includes a butting portion which butts the shaft and which has a Vickers hardness Hv not greater than about 450 and the shaft includes a butting portion which butts the torsion bar and which has a Vickers hardness Hv not greater than about 300.
The present inventors had conducted an experiment on the two members one of which is a torsion bar which is used in a power steering device of an automotive vehicle and the other of which is a shaft which is also used in the power steering device and which is press-fitted in the torsion bar. The experiment results show, as will be described in detail later, that in the case where the torsion bar includes a butting portion which butts the shaft and which has a Vickers hardness Hv not greater than about 450 and the shaft includes a butting portion which butts the torsion bar and which has a Vickers hardness Hv not greater than about 300, the amount of rebound of the two members is effectively decreased. Based on those knowledges, the method according to the present feature has been developed.
(7) An inertia-press-fit method according to any one of the features (1) to (5), wherein at least one of the two members includes a butting portion which butts the other member and which has a Vickers hardness Hv not greater than about 450.
(8) An inertia-press-fit method according to any one of the features (1) to (5), wherein at least one of the two members includes a butting portion which butts the other member and which has a Vickers hardness Hv not greater than about 400.
(9) An inertia-press-fit method according to any one of the features (1) to (5), wherein at least one of the two members includes a butting portion which butts the other member and which has a Vickers hardness Hv not greater than about 350.
(10) An inertia-press-fit method according to any one of the features (1) to (5), wherein at least one of the two members includes a butting portion which butts the other member and which has a Vickers hardness Hv not greater than about 300.
(11) An inertia-press-fit method according to any one of the features (1) to (10), wherein one of the two members includes a fitting recessed portion having a fitting hole with a bottom surface and the other member includes a fitting projecting portion which is interference-fitted in the fitting hole, and wherein the fitting projecting portion is press-fitted in the fitting hole to butt the bottom surface thereof.
According to the present feature, a portion of the fitting recessed portion that provides the bottom surface of the fitting hole and an end portion of the fitting projecting portion are utilized as respective butting portions of the two members. Thus, the two members need not be formed with respective exclusive butting portions such as stepped portions, flanges, etc.
(12) An inertia-press-fit method according to the feature (11), wherein the fitting projecting portion has an end surface and a projection which projects from the end surface and which has a diameter smaller than that of the fitting projecting portion, and wherein the projection is caused to butt the bottom surface of the fitting hole.
Thus, the projection whose diameter is smaller than that of the fitting projecting portion is caused to butt the bottom surface of the fitting hole. Since the area of butting of the projection and the bottom surface of the fitting hole is small, at least one of the projection and the bottom surface is likely to plastically deform. In addition, since a compressing force is transmitted via the projection to a central portion of the fitting projecting portion and accordingly the central portion is plastically deformed, the diameter of the fitting projecting portion increases, which leads to increasing substantially a press-fit dimension (i.e., interference-fit dimension) of the fitting projecting portion with respect to the fitting recessed portion, thereby increasing a strength with which the projecting and recessed portions are bonded with each other. This effect is very significant in the case where the hardness of the central portion of the fitting projecting portion is lower than that of an outer peripheral portion thereof. However, according to each of the features (1) to (11) and (15) to (21), it is not essentially required that the end surface of the fitting projecting portion have the projection whose diameter is smaller than that of the projecting portion. For example, if the plastic deformation of the end portion of the fitting projecting portion and/or the bottom portion of the fitting recessed portion results in absorbing sufficiently the excessive energy and thereby decreasing or preventing the rebounding, it is not needed to provide the projection.
(13) An inertia-press-fit method according to the feature (12), wherein the bottom surface of the fitting recessed portion has a recess formed in a central portion thereof, and wherein an outer peripheral portion of the projection is caused to butt a portion of the bottom surface that surrounds the recess.
Since the recess does not butt the projection, the area of butting of the projection and the bottom surface decreases as such, which contributes to causing the plastic deformation to occur more easily.
(14) An inertia-press-fit method according to the feature (12), wherein the bottom surface of the fitting recessed portion has a conical recess formed in a central portion thereof, and wherein an outer peripheral portion of the projection is caused to butt a conical surface of the conical recess.
When the outer peripheral portion of the projection butts the conical surface of the conical recess, the outer portion bites into the conical surface while the outer portion itself plastically deforms on one hand and it plastically deforms, on the other hand, a portion of the bottom surface that defines the conical surface. This is an anchoring effect, which leads to increasing the strength with which the two members are bonded with each other. The outer peripheral portion of the projection may be chamfered, and the chamfered surface may be caused to butt the conical surface. Alternatively, a non-chamfered, angular outer peripheral portion of the projection may be caused to butt the conical surface. However, in the latter case, it is preferred that the projection and/or the conical surface be not formed of a brittle material such as cast iron.
(15) An inertia-press-fit method according to any one of the features (1) to (14), wherein the plastic deformation is caused such that a maximal value of a rebound-caused deviation amount of an actual press-fit-end position from a normal press-fit-end position where the two members are in close contact with each other is not greater than a reference amount (i.e., the upper-limit value of a permissible range).
As will be described in detail in connection with the preferred embodiments of the invention, when at least one of respective butting portions of the two members that butt each other plastically deforms, the rebound amount (i.e., the deviation amount of the actual press-fit-end position from the normal press-fit-end position) increases once as the kinetic energy applied to at least one of the two members increases. However, as the kinetic energy further increases, the rebound amount decreases and eventually takes a negative value. That is, from the state in which respective butting surfaces of the two members are in close contact with each other, the two members are further deeply press-fitted with each other so that the deviation amount of the actual press-fit-end position from the normal press-fit-end position takes a negative value. Thus, the rebound amount takes a maximal point in a graph whose axis of abscissa represents kinetic energy and whose axis of ordinate represents rebound amount. The greater the plastic-deformation capabilities of the respective butting portions of the two members are, the smaller the maximal value is. Therefore, if the sum of the respective plastic-deformation capabilities of the respective butting portions of the two members is not smaller than a reference value, the maximal value of the rebound amount can fall within a permissible range. In this case, even if the kinetic energy applied may vary in a wide range, a fit length of the two members can fall within a permissible range. This is achieved according to the present feature. Thus, the application of the kinetic energy can be easily controlled and accordingly the production cost can be reduced. According to the present feature, it is preferred that the maximal value of the deviation amount be not greater than 0.5 mm, more preferably not greater than 0.3 mm, and most preferably not greater than 0.2 mm. In addition, it is preferred that each of the present feature (15) and the following features (16) to (18) be combined with a feature wherein a projection projects from an end surface of the fitting projecting portion.
(16) An inertia-press-fit method according to any one of the features (1) to (15), wherein one of the two members includes a fitting recessed portion and the other member includes a fitting projecting portion which is press-fitted in the fitting recessed portion, at least the fitting projecting portion being subjected to the plastic deformation, and wherein the two members are press-fitted with each other in a deviation-amount-increase-rate decrease range in which a rate of increase of a rebound-caused deviation amount of an actual press-fit-end position from a reference press-fit-end position where the two members are in close contact with each other decreases as the kinetic energy increases.
As described above, the rebound-caused deviation amount of the press-fit-end position is closely related to the plastic deformation of the respective butting portions of the two members. When the deviation amount falls in the deviation-amount-increase-rate decrease range, the fitting projecting portion substantially plastically deforms, which leads to increasing substantially the strength with which the two members are bonded with each other. The two-member-bonding-strength increasing effect according to each of the features (16) to (18) may be obtained in addition to the press-fit-end-position stabilizing effect (under the condition that the two effects can be simultaneously obtained), or may be obtained independent of the latter effect.
(17) An inertia-press-fit method according to any one of the features (1) to (15), wherein one of the two members includes a fitting recessed portion and the other member includes a fitting projecting portion which is press-fitted in the fitting recessed portion, at least the fitting projecting portion being subjected to the plastic deformation, and wherein the two members are press-fitted with each other in a deviation-amount decrease range in which a rebound-caused deviation amount of an actual press-fit-end position from a normal press-fit-end position where the two members are in close contact with each other decreases as the kinetic energy increases.
In the deviation-amount decrease range, a more significant bonding-strength increasing effect can be obtained.
(18) An inertia-press-fit method according to the feature (17), wherein the two members are press-fitted with each other in the deviation-amount decrease range and simultaneously in a range in which the rebound-caused deviation amount is not greater than a reference amount.
According to the present feature, it is possible to obtain both the bonding-strength increasing effect owing to the plastic deformation and the fit-length stabilizing effect. Even if the maximal value of the deviation amount may be greater than the reference amount, the deviation amount may be smaller, in respective ranges on both sides of the maximal point, than the reference amount. That is, the deviation amount can be controlled to an amount not greater than the reference amount, by controlling the kinetic energy to be applied, to a value belonging to either one of the above two ranges. On the other hand, the bonding-strength increasing effect owing to the plastic deformation of the fitting projecting portion can be obtained in any portion of a kinetic-energy range greater than the maximal point and yet with a sufficiently high degree. However, in a kinetic-energy range smaller than the maximal point, the same effect may, or may not, be obtained.
(19) An inertia-press-fit method of applying a kinetic energy to at least one of two members in a direction which causes the two members to approach each other, and press-fitting the two members with each other, characterized by
causing the two members to butt each other via a buffer member to define a relative position of the two members at a time when the press-fitting ends and, immediately after the butting of the two members via the buffer member, causing the buffer member to plastically deform and thereby absorb a remaining kinetic energy of the one member.
In the present method, the kinetic energy which remains on at least one of the two members immediately after the butting of the two members via the buffer member, is absorbed into plastic-deformation energy by the buffer member. Therefore, in the present method, even if the kinetic energy may remain immediately after the butting of the two members via the buffer member, an actual press-fit-end position is not largely deviated from a normal position.
In addition, in the present method, the energy-absorbing characteristic of the buffer member can be designed independent of respective natures of respective materials of the two members, the excessive energy can be effectively absorbed without needing to limiting the natures of materials of the two members.
The present feature wherein the energy-absorbing characteristic of the buffer member is utilized may be combined with the features (15) to (18) in each of which the elastic deformation of the respective butting portions of the two members is utilized. The following two features are representatives of those features.
(20) An inertia-press-fit method according to the feature (19), wherein the plastic deformation is caused such that a maximal value of a rebound-caused deviation amount of an actual press-fit-end position from a normal press-fit-end position where the two members are in close contact with each other is not greater than a reference amount (i.e., the upper-limit value of a permissible range).
(21) An inertia-press-fit method according to the feature (19) or (20), wherein one of the two members includes a fitting recessed portion and the other member includes a fitting projecting portion which is press-fitted in the fitting recessed portion, the buffer member having an outer diameter smaller than that of the fitting projecting portion, and wherein the two members are press-fitted with each other in a deviation-amount decrease range in which a rebound-caused deviation amount of an actual press-fit-end position from a normal press-fit-end position where the two members are in close contact with each other decreases as the kinetic energy increases.
The buffer member operates like the above-indicated projection formed on the end surface of the fitting projecting portion, and the projecting portion plastically deforms to increase the bonding-strength increasing effect.
(22) An inertia-press-fit method according to any one of the features (1), (2), (4), (11), and (15) to (18), wherein, at at least an initial phase of the butting of the two members, a butting projection and a butting portion which is less harder than the butting projection butt each other, so that the butting projection bites into the butting portion.
According to the present feature, at at least the initial phase of the butting of the two members, a butting projection which is provided as part of one of the two members and a butting portion which is provided as part of the other member butt each other, so that the butting projection bites into the butting portion that is less harder than the butting projection. Consequently the butting portion is plastically deformed so that at least a portion of the kinetic energy which remains on at least one of the two members is absorbed. In the present method, even if some kinetic energy may remain on at least one of the two members at the initial phase of the butting of the two members, an actual press-fit-end position is not largely deviated from a normal position.
(23) An inertia-press-fit method according to the feature (22), wherein one of the two members includes a fitting recessed portion having a fitting hole with a bottom surface and the other member includes a fitting projecting portion which is interference-fitted in the fitting hole, and wherein the butting projection comprises a butting projection projecting from a portion of the bottom surface of the fitting hole and the butting portion comprises an end surface of the fitting projecting portion.
Since the butting projection projects from a portion of the bottom surface of the fitting hole, the area of butting of the butting projection with the end surface of the fitting projecting portion as the butting portion is small as compared with the case where the entire bottom surface of the fitting hole and the entire end surface of the projecting portion butt each other. Thus, the plastic deformation more easily occurs. In addition, since the end portion of the fitting projecting portion plastically deforms, the external dimension of the projecting portion increases so that the press-fit dimension (i.e., interference-fit dimension) of the projecting portion with respect to the recessed portion substantially increases. Therefore, the strength with which the projecting and recessed portions are bonded with each other is increased.
(24) An inertia-press-fit method according to the feature (22), wherein one of the two members includes a fitting recessed portion having a fitting hole with a bottom surface and the other member includes a fitting projecting portion which is interference-fitted in the fitting hole, and wherein the butting projection comprises a hard member which is provided between the bottom surface of the fitting hole and an end surface of the fitting projecting portion and which is harder than at least one of the bottom surface and the end surface.
The present feature is preferably employed in the case where it is difficult, in view of structure or material, to form a projection projecting from the bottom surface of the fitting hole or the end surface of the fitting projecting portion, or increase the hardness of the bottom surface or the end surface. Since the butting projection is provided by the hard member provided between the bottom surface or the end surface, the degree of freedom regarding the selection of respective materials of the two members and/or the determination of hardness difference of the hard member and at least one of the two members, is increased. The hard member may be temporarily attached, by adhesion, sticking, soldering, etc., to one of the two members that is kept still, or may be strongly fixed, by, e.g., blazing, to either one of the two members. Alternatively, depending upon the shape of the hard member, the hard member may be held in a holding hole which is formed in one of the two members. In the case where the holding hole has a shape which can stably hold the hard member, the above-indicated temporary attaching or the fixing blazing may be omitted.
(25) An inertia-press-fit method according to the feature (24), wherein the hard member is a steel ball.
According to the present feature, the hard member can be easily provided by a cheap and highly hard member. In the case where a commercially available steel ball is used, the cost is more largely reduced.
(26) An inertia-press-fit method according to any one of the features (23) to (26), wherein the fitting recessed portion has an undercut which is formed in a portion of an inner circumferential surface of the fitting hole that is near the bottom surface, and whose diameter is greater than that of a remaining portion of the inner circumferential surface, and wherein when the butting projection bites into the end surface of the fitting projecting portion, an end portion of the fitting projecting portion is forced to expand outward and thereby engage the undercut of the fitting recessed portion.
In the present method, the material of the end portion of the fitting projecting portion that is expanded outward by the butting projection is allowed, by the undercut, to flow. Thus, the end portion of the fitting projecting portion is more easily plastically deformed, and the excessive energy at the initial phase of the butting is effectively absorbed. In addition, since the outward expanded portion of the fitting projecting portion engages the undercut, the fitting projecting portion is effectively prevented from coming off the fitting recessed portion, and the strength with which the two members are bonded with each other is increased. The undercut may be provided in the form of a full-annular groove, or a partly formed recess such as a petal-like recess.
(27) An inertia-press-fit method according to the feature (22), wherein one of the two members includes a fitting recessed portion having a fitting hole with a bottom surface and the other member includes a fitting projecting portion which is interference-fitted in the fitting hole, and wherein the butting projection comprises a butting projection projecting from a portion of an end surface of the fitting projecting portion and the butting portion comprises the bottom surface of the fitting hole, the butting projection being caused to bite into the bottom surface of the fitting hole.
According to the present feature, at least a portion of the kinetic energy which remains at the initial phase of the butting is absorbed by the plastic deformation of the bottom surface of the fitting recessed portion, so that the press-fit-end position of the two members is stabilized.
(28) An inertia-press-fit method of applying a kinetic energy to at least one of two members in a direction which causes the two members to approach each other, and press-fitting the two members with each other, characterized in that
one of the two members includes a fitting recessed portion having a fitting through-hole, the other member includes a fitting projecting portion which is interference-fitted in the fitting through-hole, and, in a state in which a third member is inserted in the fitting through-hole to an intermediate position thereof from one of two end openings thereof that is opposite to the other end opening in which the fitting projecting portion is to be press-fitted, the fitting projecting portion is press-fitted in the fitting through-hole to butt the third member.
Since the fitting recessed portion has the fitting through-hole, the third member can be used to define the press-fit-end position of the two members. Since, in the state in which the third member is inserted in the fitting through-hole from one of the two end openings thereof that is opposite to the other end opening in which the fitting projecting portion is to be press-fitted, the fitting projecting portion is press-fitted in the fitting through-hole to butt the third member, the press-fit-end position of the fitting projecting portion can be selected at any desired position. Therefore, the press-fit-end position of the two members can be selected at any desired position by selecting a depth to which the third member is inserted in the fitting through-hole.
(29) An inertia-press-fit method according to the feature (28), wherein a butting projection which is harder than an end surface of the fitting projecting portion is formed on an end surface of a portion of the third member that is inserted in the fitting through-hole, and wherein the butting projection is caused to bite into the end surface of the fitting projecting portion.
Since the butting projection formed on the third member bites into the end surface of the fitting projecting portion, at least a portion of the kinetic energy remaining on at least one of the two members is absorbed by the plastic deformation of the end portion of the fitting projecting portion. Thus, the rebounding of the fitting projecting portion and the third member is prevented or reduced, and an actual press-fit-end position is not largely deviated from a prescribed position.
(30) An inertia-press-fit method according to the feature (29), wherein the fitting recessed portion has an annular groove which is formed in a portion of an inner circumferential surface of the fitting through-hole that is adjacent to the end surface of the third member, and wherein a portion of the fitting projecting portion that is forced, by the biting of the butting projection, to expand outward engages the annular groove.
According to the present feature, the end portion of the fitting projecting portion is likely to plastically deform, and the portion of the fitting projecting portion that is forced to expand outward is caused to engage the annular groove. Therefore, the strength with which the two members are bonded with each other is increased.